1. Field of the Invention
This invention relates to mask used in a semiconductor fabrication process, and more particularly, to a mask that is compatible with steppers of different manufacturers.
2. Description of Related Art
Photolithography is a process commonly used in patterning and implantation processes. Generally, the number of photolithography processes, the number of required masks, represents the difficulty of fabricating a semiconductor device. In addition, the development and improvement of a higher-resolution mask helps the achievement on reducing the linewidth of a semiconductor fabrication process, that is, increases the integration of a semiconductor device.
The principle of photolithography basically includes coating a photoresist layer on a wafer, and selectively exposing the photoresist layer by the light pass through a mask to transfer the pattern on the mask onto the photoresist layer.
Besides a light source, a photolithography process also required a mask having a predetermined pattern thereon. The main body of a mask is a piece of planer and transparent reticle glass. The predetermined pattern is formed on the piece of reticle glass by coating a chromium thin film thereon, wherein the thickness of the chromium thin film is about several hundred angstroms. An additional chromium dioxide thin film of about 200 .ANG. thick is optionally coated on the chromium thin film to prevent the reflection of incident light.
Conventionally, there are three basic types of exposure process including contact, proximity, and projection. Among the foregoing exposure processes, the projection-type exposure process is advantaged on protecting the pattern on the mask, and providing a higher resolution, wherein the projection-type exposure process is normally performed by using a stepper.
Since a fabrication process sometimes has steppers of different manufacturers involved for increasing the yield, and different manufacturers use masks of different standards, so several sets of masks are required for the same processes performed on the steppers of different manufacturers. Hence, the cost of preparing masks is increased, and the fabrication flow becomes complicated as well.